The present invention relates to a screen printing machine for performing a through hole printing to print an electric conductor onto the surface of a substrate and the inner surface of a through hole formed in the substrate.
Recently, it has become important to develop high-density packaging techniques to provide electronic equipment with more functions. Specially, in order to effectively utilize the limited surface area of a substrate, one method is employed in which electronic parts are mounted on the upper and lower surfaces of the substrate so that some parts mounted on the upper and lower surfaces are electrically short-circuited. A through hole printing method for printing an electric conductor onto the inner surfaces of through holes formed in the substrate is employed as one of the methods for short-circuiting the parts. The through hole printing method is so arranged that suction is exerted through the through holes previously formed in the substrate in printing the electric conductor onto the surface of the substrate to print and attach the conductor onto the inner surfaces of the through holes. At this time, it is necessary to adjust the suction condition in accordance with the position, number, and size of the through holes of the substrate. Therefore, conventionally, a known machine for adjusting the suction condition is, as shown in FIG. 4, so constructed that two kinds of suction conduits 30a and 30b, that is, a suction conduit 30a for a low flow rate and a suction conduit 30b for a high flow rate, with respective manual throttle valves 31a and 31b, are provided between a flow meter 19 and a changeover valve 32 to change the conduits 30a and 30b by the valve 32. Furthermore, in FIG. 4, reference numeral 2 denotes a table, 3 a substrate position adjusting member arranged on the table 2, 4 a screen plate capable of being lowered onto the table 2, 5 a squeegee movable on the plate 4, 6 paste, 8 a substrate to be put on the table 2 to be printed, 9 through holes previously formed in the substrate 8, 10 a suction unit under the table 2, 11 support pins in the suction unit 10, 19 a flow meter connected with the suction unit 10 through a pipe 35, 25 a changeover valve for changing between conduits 23 and 24, 26 a pipe, and 27 a blower functioning as a suction source by producing vacuum. The changeover valves 32 and 25 are connected with each other through the conduit 24. In the machine, the change-over valve 32 changes between the conduits 30a and 30b in accordance with the position, number, and size of the through holes of the substrate so as to adjust the suction condition.
However, in the machine, it is necessary to provide several kinds of suction conduits for one suction unit, resulting in complex construction. When the suction conduits 30a and 30b are changed over by the changeover valve 32, the suction flow rate is immediately changed, resulting in rapid change of the degree of vacuum in the suction unit which has an adverse effect on the through hole printing.
Additionally, when the screen printing is performed on the substrate, the substrate is accurately positioned with respect to a screen plate and then paste previously supplied on the plate is transmitted through the plate by movement of the squeegee to coat the paste onto the surface of the substrate and the inner surfaces of the through holes. Thus, in this through hole printing method, it is necessary to simultaneously perform the coating and the suction operations.
In this case, the total cross-sectional area of the holes at the time the through hole printing is first performed differs from the cross-sectional area at the time the printing is last performed because of the paste printed on the substrate. Thus, when the suction operation is performed at a constant flow rate (Q), as shown in FIGS. 5A-5D, the degree of vacuum is lower at the start of the printing, so that it is difficult to sufficiently suck paste through the through holes. The degree of vacuum is gradually increased as the total cross-sectional area of the through holes is reduced due to the paste adhering to the walls thereof, and then the degree of vacuum reaches a specified value which may remove the paste adhered to the inner surfaces of the through holes from the inner surfaces by the suction, resulting in lower reliability of the printing.
As shown in FIGS. 6A-6D, in order to overcome these disadvantages, it is recognized by the inventors that it is necessary to delicately adjust the degree of vacuum by the adjustment of the suction flow rate so that the through holes firstly printed and those lastly printed will be uniformly printed.